Electronic devices are in use today to add reverberation to signals used in sound reproduction. Such artificial reverberation units are intended to act on these sound signals to achieve the echo or reverberation effect of large rooms or concert halls. In a system designed for home high-fidelity sound reproduction equipment, this realistic reproduction of reverberation must be achieved at a moderate cost in order for such a system to be economically feasible. Several reverberation systems are available for use in high-fidelity systems, but these systems suffer from a lack of realism and a characteristic artificiality in the final sound which has come to be commonly associated with reverberation systems or employ, for example, reverberation chambers and plates which are not practical for use in home high-fidelity systems because of their size and expense.
Several features of a concert hall must be provided for in a reverberation system if the final sound output is to realistically simulate the sound heard in a concert hall or other performance room. An important such feature required in a reverberation system is reverberation time, defined as the time for the sound intensity to decay by 60 dB. While the ideal reverberation time varies, depending on the particular sound being reproduced, a typical range for reverberation times might be from 0.5 to 2.0 seconds.
Another very important reverberation feature is the interval between reflections. While a concrete tunnel may have a reverberation time equal to a concert hall, there is a very distinct difference between sounds reproduced in each. In concert halls with good acoustics, sound reflections follow many different paths having many different intervals between each of the reflections or echoes. The result is that the reflections effectively occur at a large number of random times producing a high density of echoes, such that individual reflections cannot be discerned by the human ear.
In order for a system to provide realistic reverberation in an input signal, it must simulate very closely the response of an actual concert hall with respect to each of the parameters mentioned above.
Beside reverberation, another factor important in sound reproduction systems is the illusion of directionality in the reproduced sound. It is well known that a stereo sound reproduction system reproduces sound much more realistically than a single channel system due to the apparent directionality of the reproduced sound. Recently, quadraphonic sound systems having four separate sources of reproduced sound have been gaining increasing acceptance because of their further improvements in directionality. However, much of the recorded material available today has only two signal channels. Because of this, there is a demand for systems which can provide signals for driving quadraphonic speakers from a single stereo input signal pair.